[dcl.attr.grammar] Fix typo "appeartain" -> "appertain" (#6420)

Author: notadragon

source/declarations.tex

@@ -8651,7 +8651,7 @@
 appertains to a friend declaration\iref{class.friend}, that declaration shall be a
 definition.
 \begin{note}
-An \grammarterm{attribute-specifier-seq} cannot appeartain to
+An \grammarterm{attribute-specifier-seq} cannot appertain to
 an explicit instantiation\iref{temp.explicit}.
 \end{note}